fix: multiple selection count

- Only add files to selected list that arent on it.
- Only shift key select when there are selected files.

frontend/src/components/files/ListingItem.vue

@@ -129,7 +129,7 @@ export default {
         return
       }
 
-      if (event.shiftKey) {
+      if (event.shiftKey && this.selected.length > 0) {
         let fi = 0
         let la = 0
 
@@ -142,7 +142,9 @@ export default {
         }
 
         for (; fi <= la; fi++) {
-          this.addSelected(fi)
+          if (this.$store.state.selected.indexOf(fi) == -1) {
+            this.addSelected(fi)
+          }
         }
 
         return